For instance, an electric power steering system, which employs a speed reduction mechanism comprised of a worm wheel provided on the outer periphery of a steering shaft and a wormshaft, which is in meshed-engagement with the worm wheel and whose one axial end is connected to an electric motor, is well known as a typical power steering system. The electric power steering system is configured to assist a steering force by a driving force, produced by the electric motor and transmitted through the speed reduction mechanism, in response to a measure of torque in the steering shaft.
In such an electric power assisted steering system, in the presence of a large backlash between the toothed portion of the worm wheel and the toothed portion of the wormshaft meshing each other, there exists a problem with hammering noise generated between the worm wheel and the wormshaft meshing each other due to vibrations transmitted through road wheels. To avoid this problem, the following patent document 1 has proposed and disclosed a power steering system, capable of adjusting a backlash of a speed reduction mechanism.
This type of power steering system includes a bearing fitted onto the outer periphery of the other axial end of the wormshaft, which wormshaft is housed in a shaft housing portion defined in a housing in such a manner as to penetrate the inside of the housing for rotatably supporting the other axial end of the wormshaft, a preloading member adapted to force the wormshaft in the direction of the one axial end of the wormshaft (that is, toward the electric-motor side) through the bearing, an adjusting screw threadably engaged with the opening end of the shaft housing portion to which the other axial end of the wormshaft faces, such that the adjusting screw forces the preloading member toward the bearing for adjusting the magnitude of a preload of the preloading member by turning the adjusting screw, and a resilient: biasing means installed on the outer circumference of the bearing and adapted to act on the bearing to force the other axial end of the wormshaft in the direction perpendicular to the axis of the wormshaft through the bearing, so as to bias the other end of the wormshaft in the direction of meshed-engagement with the worm wheel. The previously-noted biasing means is comprised of an ring, which is fitted and retained in an annular groove, cut or formed in the inner peripheral surface of the opening end of the shaft housing portion. The O ring is laid out to face the outer peripheral surface of the bearing. The center of the annular groove when viewed in the radial direction is slightly offset from the center of the shaft housing portion when viewed in the radial direction, toward the worm-wheel side, thus enabling the other axial end of the wormshaft to be biased toward the worm-wheel side through the bearing by virtue of an elastic force of the O ring.
With the previously-noted arrangement, the magnitude of a preload of the preloading member can be adjusted by forward/backward rotary motion of the adjusting screw. Additionally, the elastic force of the O ring can be adjusted by selection of the cross-sectional dimension of the O ring and the eccentricity of the center of the groove when viewed in the radial direction with respect to the center of the shaft housing portion when viewed in the radial direction, and whereby the desired amount of biased displacement of the wormshaft in the direction of meshed-engagement with the worm wheel can be adjusted. In this manner, it is possible to realize an appropriate backlash between the toothed portion of the worm wheel and the toothed portion of the wormshaft meshing each other. Patent document 1: JP2002-518242TT (see FIG. 1 thereof)